1. Field of the Invention
The invention relates to a position detector for the movable element of a fluid-operated actuator.
2. Description of Related Art
It is frequently necessary, in the case of pneumatic or hydraulic piston-cylinder units, to detect the front or rear end of the piston or an intermediate position reached by the piston. To this effect, mechanical limit switches may be used. However, the accuracy of such limit switches is insufficient, and their life time in terms of switching operations is relatively low. Further, the service life of such mechanical switches is reduced due to the mechanical wear of parts contained in such switches.
Known limit switches for hydraulic or pneumatic cylinders have been of the proximity type. These known limit switches comprise an electric member which is responsive to the approximation of an electrically conductive element. The electrically conductive element may, for instance, consist of the coil of an oscillator circuit. Upon approximation of a metal element, the oscillator circuit is attenuated, resulting in a change or cessation of the circuit oscillation.
The oscillator circuit coil is provided in a cylindrical probe which is embedded in a bore of a wall of the hydraulic or pneumatic cylinder. The bore communicates with the cylinder chamber. If the piston approaches the end of the probe which includes the electric element, the latter contactlessly reacts on the approximation of the piston, thus indicating that the piston has reached the position to be detected.
However, in the case of known contactless piston position detectors, the reaction distance of the probe is limited to only about 2 mm. Therefore, it is rather important for the end of the probe to be arranged very close to the cylinder chamber. Varying the wall thickness of the cylinder wall entails varying the probe length. On the other hand, the bore of the cylinder wall must be sealed against the high hydraulic or pneumatic pressures.
Manufacturers of contactless piston position detectors therefore generally sell a plurality of models which only differ by the length of the cylindrical probe, which is fixed to the holder and which is sealingly secured at the outside of the pressure cylinder and projects from the holder to such an extent that its end is flush with the inner surface of the cylinder wall. The need to provide a great number of position detectors which only differ by their respective probe lengths is extraordinarily expensive from an engineering and storage point of view.
Moreover, it is quite difficult to always determine by experimentation the exact probe length required for each individual purpose. In fact, sometimes the probe of a position detector may be too long, while the model of the following line is too short for the respective requirement. In such cases, a washer is required to be placed between the cylinder wall and the holder of the position detector, to thereby permit use of the position detector having the longer probe.
It is an object of the present invention to provide a position detector which is applicable within a large range of wall thicknesses of the pressure-operated actuator.